1. Field of the Invention
The present invention relates to an MEMS sensor and a production method thereof.
2. Description of Related Art
In recent years, an MEMS sensor such as an Si (silicon) microphone produced by MEMS (Micro Electro Mechanical Systems) has been employed as a microphone loaded on a portable telephone or the like.
FIGS. 3A to 3K are schematic sectional views successively showing the steps of producing a conventional Si microphone 101. The method of producing the conventional Si microphone 101 and the structure thereof are now described with reference to FIGS. 3A to 3K.
In order to produce the conventional Si microphone 101, SiO2 (silicon oxide) is deposited on the overall surfaces of an Si wafer W2 by thermal oxidation, as shown in FIG. 3A. Thus, a lower sacrificial layer 111 made of SiO2 is formed on the upper surface of the Si wafer W2. Further, an SiO2 film 119 is formed on the lower surface of the Si wafer W2.
Then, a photoresist film 120 having holes 121 of a prescribed pattern is formed on the upper surface of the lower sacrificial layer 111, as shown in FIG. 3B. The lower sacrificial layer 111 is etched through the photoresist film 120 employed as a mask, whereby a plurality of (four in FIG. 3C) recesses 112 are formed in the upper surface of the lower sacrificial layer 111, as shown in FIG. 3C. After the formation of the recesses 112, the photoresist film 120 is removed.
Then, polysilicon is deposited on the overall surfaces of the lower sacrificial layer 111 and the SiO2 film 119 by LPCVD (Low Pressure Chemical Vapor Deposition). The polysilicon film covering the lower sacrificial layer 111 is doped with phosphorus, and portions of this polysilicon film other than that present on a prescribed region including the plurality of recesses 112 are thereafter removed by well-known photolithography and etching. Thus, a thin-film polysilicon plate 104 is formed on the prescribed region of the lower sacrificial layer 111, as shown in FIG. 3D. Further, a polysilicon film 113 is formed on the SiO2 film 119.
Then, SiO2 is deposited on the overall surfaces of the lower sacrificial layer 111 and the polysilicon plate 104 by PECVD (Plasma Enhanced Chemical Vapor Deposition). Then, unnecessary portions of the deposited SiO2 film are removed by well-known photolithography and etching. Thus, an upper sacrificial layer 114 made of SiO2 is formed on the polysilicon plate 104 and a region around the same, as shown in FIG. 3E.
Then, polysilicon is deposited on the lower sacrificial layer 111, the upper sacrificial layer 114 and the polysilicon film 113 by LPCVD (Low Pressure Chemical Vapor Deposition). Thus, the polysilicon film deposited on the polysilicon film 113 and the polysilicon film 113 are integrated into a polysilicon film 115, as shown in FIG. 3F. On the other hand, the polysilicon film deposited on the lower sacrificial layer 111 and the upper sacrificial layer 114 is doped with phosphorus, and thereafter patterned by well-known photolithography and etching. Thus, a back plate 105 having a large number of holes 106 is formed on the upper sacrificial layer 114, as shown in FIG. 3F.
Then, a photoresist film 122 having holes 123 of a prescribed pattern is formed on the overall region of the upper sacrificial layer 114 including the back plate 105, as shown in FIG. 3G. Then, the upper sacrificial layer 114 is etched through the photoresist film 122 employed as a mask. Thus, a plurality of (four in FIG. 3H) recesses 117 are formed in the upper surface of the upper sacrificial layer 114 while unnecessary portions (other than the portion opposed to the upper sacrificial layer 114) of the lower sacrificial layer 111 are removed, as shown in FIG. 3H. After the formation of the recesses 117, the photoresist film 122 is removed.
Then, the polysilicon film 115 is removed, and an SiN (silicon nitride) film 107 is thereafter formed on the upper region of the Si wafer W2 by PECVD, as shown in FIG. 3I.
Then, holes 118 communicating with the holes 106 of the back plate 105 respectively are formed in the SiN film 107 by well-known photolithography and etching, as shown in FIG. 3J. Thus, the upper sacrificial layer 114 is partially exposed through the holes 106 and 118. Further, an opening is formed in the portion of the SiO2 film 119 opposed to the polysilicon plate 104 by well-known photolithography and etching. Then, the Si wafer W2 is etched through this opening, so that a through-hole 103 is formed in the Si wafer W2. Consequently, the lower sacrificial layer 111 is partially exposed through the through-hole 103.
Then, an etching solution capable of etching SiO2 is supplied through the through-hole 103 and the holes 106 and 118, to wet-etch the upper sacrificial layer 114 and the lower sacrificial layer 111. Thus, the polysilicon plate 104 floats up from the upper surface of the Si wafer W2 while a cavity of a small interval is formed between the polysilicon plate and the back plate 105, as shown in FIG. 3K.
Thereafter the Si wafer W2 is divided into an Si substrate 102 of each device size, whereby the Si microphone is obtained with the polysilicon plate 104 and the back plate 105 opposed to each other through the cavity 110. Portions of the SiN film 107 having entered the recesses 117 of the upper sacrificial layer 114 become protrusions 109 protruding toward the polysilicon plate 104, to function as stoppers for preventing the polysilicon plate 104 and the back plate 105 from adhesion and a short circuit. Further, portions of the polysilicon plate 104 having entered the recesses 112 of the lower sacrificial layer 111 become protrusions 108 protruding toward the upper surface of the Si wafer W2, to function as stoppers for preventing the Si substrate 102 and the polysilicon plate 104 from adhesion. The polysilicon plate 104 and the back plate 105 are supported by unshown wires.
The polysilicon plate 104 and the back plate 105 form a capacitor opposed through the cavity 110. When a sound pressure (sound wave) is input in the Si microphone 101 from above the back plate 105, the polysilicon plate 104 vibrates due to this sound pressure, and the capacitor outputs an electric signal responsive to a change of the capacitance of the capacitor resulting from this vibration of the polysilicon plate 104.
When the thin-film polysilicon plate 104 vibrates or is electrostatically attracted to the Si substrate 102 and comes into contact with the Si substrate 102 over a wide contact area, the polysilicon plate 104 and the Si substrate 102 may adhere to each other. Therefore, the plurality of protrusions 108 are formed on the polysilicon plate 104. Thus, the protrusions 108 come into contact with the Si substrate 102 when the polysilicon plate 104 approaches the Si substrate 102, whereby the polysilicon plate 104 and the Si substrate 102 can be prevented from adhesion.
In order to form the protrusions 108, however, the step (see FIG. 3B) of forming the photoresist film 120 having the holes 121 on the upper surface of the lower sacrificial layer 111 and the step (see FIG. 3C) of forming the recesses 112 in the upper surface of the lower sacrificial layer 111 by etching through the photoresist film 120 serving as a mask are required, leading to a long time and much labor for the formation of the protrusions.